The water gas shift (WGS) reaction, which can be represented by the equation CO+H2O→CO2+H2, is a powerful tool for efficiently producing hydrogen-containing product gases from carbonaceous sources. Typically a feed containing carbon monoxide obtained for instance from gasification (partial combustion) of coal or biomass, and water (steam) is fed into a WGS reactor to obtain a mixture of hydrogen and other components, including carbon dioxide. Alternatively, methane can be used as a source of CO-containing gas for a WGS process, either by catalytic partial oxidation (CPO) according to the equation: CH4+½O2→CO+2H2 or by steam reforming according to the equation: CH4+H2O→CO+3H2, or by mixed modes thereof.
For a maximum conversion to hydrogen, an excess of water is required, and therefore conventional WGS processes use a H2O:CO molar feed ratio of well over 1, typically around 2 or higher. Removal of CO2 during or after the WGS reaction then results in a high purity hydrogen product. Conventionally, pure H2 is obtained by removal, not only of CO2, but also CO, CH4 and N2 via e.g. PSA. In the conventional process, CO is also present due to the thermodynamically limited CO conversion.
WO 2010/059052 and Van Dijk et al., Intern. J. Greenhouse Gas Control, 5 (2011), 505-511), describe a sorption-enhanced water gas shift (SEWGS) process to produce hydrogen and carbon dioxide as well as hydrogen sulphide, wherein the carbon dioxide and hydrogen sulphide are adsorbed onto an alkali promoted hydrotalcite adsorbent. The carbon dioxide and hydrogen sulphide are subsequently simultaneously removed from the adsorbent.
WGS processes are typically carried out at high pressures and at relatively high temperatures (200-600° C.). The supply of high-pressure steam is relatively expensive and detracts from the total process efficiency. This is particularly relevant for CPO (catalytic partial oxidation) and gasification feeds, since these feeds do not contain sufficient steam. Therefore there is a case for lowering the level of steam in a WGS feed, while retaining high conversion levels of CO to H2.
WO 2011/000792 (Shell) discloses a process for producing hydrogen-rich gas mixtures involving a WGS reaction wherein a low H2O/CO molar ratio of 0.2-0.9 is used in the feed mixture. As a result of this low molar ratio, the gas issuing from the WGS reactor still has a high CO content and a low H2/CO molar ratio of less than 1. This problem is overcome according to WO 2001/000792 by using multiple WGS reactors in series, wherein steam is added before each subsequent reactor. The WGS reaction proceeds at relatively low feed temperatures (190-230° C.) and requires high levels of hydrogen sulphide in the order of 2000 ppm in order to keep the WGS catalyst (Mo/Co-based) active. The H2S and the CO2 have to be removed downstream e.g. by washing with a polyethylene glycol ether. This is one of the downsides of the process of WO 2010/000792, together with the fact that, in accumulation, still about a stoichiometric amount of high pressure steam with respect to the amount of CO has to be supplied.
WO 2010/000387 (Haldor Topsøe) also deals with the problem of the high-pressure steam requirement, while avoiding methanation reactions, in high feed temperature (about 390° C.) WGS reactions. The solution proposed by WO 2010/000387 is to use an alkali-promoted zinc/alumina catalyst for the WGS reaction, as further detailed in co-pending EP 2141118. Although the catalyst stability appeared to be satisfactory, and the methanation appeared to be suppressed, no solution is provided for reaching satisfactory CO conversion levels while operating with reduced steam feeds.
Wright et al. (Energy Procedia, 4 (2011) 1147) describe the role of steam in the purging and rinsing of a SEWGS process using a steam/CO molar ratio of 1.6.
Jang et al. (Chem. Engineering Sc. 73 (2012) 431) describe a SEWGS process using a steam/CO molar ratio of 5. Using a theoretical model, they conclude that the optimum hydrogen production is achieved for the hydrogen production in a SEWGS process since the WGS reaction, and thus CO2 separation, are suppressed.